Isotopic equilibration during partial melting: an experimental test of the behaviour of Sr

Experiments using isotopically enriched, Sr doped minerals designed to test for isotopic equilibrium between source and melt during partial melting reveal that87Sr/86Sr ratios of the liquids are primarily determined by the proportions of minerals consumed and vary with the advancement of the melting reaction. The experiments were performed at 1 atm on model crustal assemblages composed of pairs of natural plagioclase (An68;87Sr/86Sr= 0.701) and synthetic fluorphlogopite doped with 90 ppm Sr having87Sr/86Sr= 4.2. SIMS traverses showed that during the initial stages of the reaction, liquids are isotopically zoned. All the analyzed melts have87Sr/86Sr markedly higher than that of the bulk starting assemblage (i.e. the source), because of the faster melting rate of fluorphlogopite. 0°C and 1250°C melting occurs above a critical temperature, wherein the dissolution rates of the crystals are controlled by diffusion of species in the melt, and reactants and reaction products are out of isotopic equilibrium. This is due to faster melt-crystal boundary migration when compared to Sr diffusion in the crystals. Equilibration is possible only if melting stops. Calculations show that total equilibration between melt and residue by Sr tracer diffusion in the crystals takes 104–106 yr (for temperature and grain size ranging, respectively, from 800°C to 1000°C, and 0.1 to 1 cm). When compared to the proposed residence time of crustal magmas at their sources, this result strongly suggests that magmas that do not reflect the bulk isotopic characteristics of their source regions can be produced.